1. Ch. 2—Key concepts
Correct identification of fossils is the basis for all subsequent interpretations and applications; an understanding of intraspecific variation is necessary for correct identification
Ontogenetic variation occurs during an individual’s lifespan
Population variation occurs among individuals within a given population
Fossils & Evolution Ch. 2

2. Ch. 2—Key terms Ontogeny; ontogenetic variation Population variation
Types of skeletal growth
Addition; accretion; molting; modification; combination
Isometric vs. allometric growth
Principle of similitude
Ecophenotypic variation
Sexual dimorphism
Fossils & Evolution Ch. 2

3. Ontogenetic variation
Ontogeny = the life history of an individual (both embryonic and post-natal)
Understanding ontogeny is important because growth stages of an individual may be so different that they are hardly recognizable as the same species
Fossils & Evolution Ch. 2

4. Types of skeletal growth
Accretion (enlargement) of existing parts
Addition of new parts
Molting
Modification
Combinations (mixed growth strategies)
Fossils & Evolution Ch. 2

5. Skeletal growth—Accretion
Accretion = adding new material to an existing shell
Allows uninterrupted use of shell and more or less continuous growth
Disadvantage is that adult shape is somewhat constrained by juvenile shape
Example: bivalve growth
Fossils & Evolution Ch. 2

6. Bivalve accretion
Fossils & Evolution Ch. 2

7. Skeletal growth—Addition of new parts
Echinoderms may grow simply by adding new plates to their calyx or new columnals to their stalk
Example: crinoid stalk
Large columnals added just beneath calyx
Smaller columnals added between larger ones
Alternation of different sizes allows increased flexibility
Fossils & Evolution Ch. 2

8. Crinoid stalk (addition)
Fossils & Evolution Ch. 2

9. Skeletal growth—Molting
Molting = periodic shedding of an exoskeleton followed by growth of a new, larger one
Advantage: Shape of adult organism not constrained by shape of juvenile stages
Disadvantages are (1) vulnerable period during the molt itself; (2) significant metabolic cost of repeatedly replacing entire skeleton
Example: trilobites
Fossils & Evolution Ch. 2

10. Trilobite molting Instars = growth stages between molts
Fossils & Evolution Ch. 2

11. Molting (cont.) Molting produces growth in instars
a series of discrete episodes
(not continuous)—Instars
from different growth
stages form distinct
morphologic clusters
instars
Fossils & Evolution Ch. 2

12. Skeletal growth—Modification
Modification = process of replacement and re-formation of skeletal material, allowing size increase as well as changes in shape and structure
Skeletal form of adult is not strongly constrained by skeletal form of juvenile
No vulnerable stage (as in molting)
Example: vertebrate bones
Fossils & Evolution Ch. 2

13. Skeletal growth—Mixed strategies
Some organisms employ combinations of growth strategies
Example: coiled cephalopod grows by accretion along leading edge of shell and also by periodic addition of septa
Fossils & Evolution Ch. 2

14. Combined growth strategy (coiled cephalopod)
continuous accretion
of new material along
leading edge of shell
periodic addition
of new septa
Fossils & Evolution Ch. 2

15. Recognizing and describing ontogenetic change
Biologists can directly observe ontogenetic change, but paleontologists cannot
Two main approaches to studying ontogenetic changes in fossil material:
Growth series of specimens representing different developmental stages (as in successive trilobite instars)
Adult specimens whose development is recorded by growth lines or newly added parts (as in bivalve example)
Fossils & Evolution Ch. 2

16. Recognizing and describing ontogenetic change
Approach depends on the kinds of fossils being studied:
Cannot use adult specimens to study ontogeny in animals that grow through molting or modification
Fossils & Evolution Ch. 2

17. Example 1: Brachiopod ontogeny
Length and width measurements performed on large (~75) population of specimens of all sizes
Plot of length vs. width suggests change in shape during growth
Small individuals are wider than long
Large individual are longer than wide
Fossils & Evolution Ch. 2

18. Brachiopod example: Length vs. width
Growth Series:
scatter of data
points suggests
change in shape
during growth
Fossils & Evolution Ch. 2

19. Example: Brachiopod ontogeny
A more definitive understanding of brachiopod ontogeny can be achieved by plotting growth curves for individual specimens (by measuring along growth lines)
Fossils & Evolution Ch. 2

20. Brachiopod example: Length vs. width
Individual ontogeny:
growth curves for single
specimens confirm
change in shape, AND
allow estimate of
variation among
individuals
Fossils & Evolution Ch. 2

21. Types of growth
Isometric = no change in shape during ontogeny (ratio between parts does not change as size increases)
Relatively uncommon
Anisometric (allometric) = change in shape during ontogeny (ratio between parts changes as size increases)
Relatively common
Fossils & Evolution Ch. 2

22. Types of growth (cont.) Consider two body parts, X and Y
As organism grows, relationship between X and Y is given as:
In isometric growth, a = 1 (linear equation)
In anisometric growth, a = 1 (curve)
Y = bXa
Fossils & Evolution Ch. 2

23. Isometric growth
Fossils & Evolution Ch. 2

24. Anisometric growth
Fossils & Evolution Ch. 2

25. Why is anisometric growth common?
Anisometric growth is necessary in most organisms because volume (body mass) increases as the cube of linear size increase
Example: bone strength is proportional to cross-sectional area of bone
As linear dimensions of bone doubles, cross-sectional area is squared, but body mass is cubed
Body weight increases faster than relative strength of supporting bones
This scaling inequality is “principle of similitude”
Fossils & Evolution Ch. 2

26. “Principle of similitude”10 20 2 2
Cross-sectional area = 16
Volume = 320
Cross-sectional area = 4
Volume = 40 4 4
Fossils & Evolution Ch. 2

27. Anisometry of pelycosaur femurs (note different shapes as well as different sizes)
Fossils & Evolution Ch. 2
decreasing size of animal

28. Population variation
Variation among individuals within a population is called population variation
Sources of population variation are:
Genetic differences among individuals
Ecophenotypic variation
Sexual dimorphism
Taphonomic effects
Fossils & Evolution Ch. 2

29. Populations
Biologic definition of population = “a group of individuals of the same species living close enough together that each individual of a given sex has a chance of mating with an individual of the other sex”
“breeding population”
Populations are characterized by a single gene pool
Gene flow occurs when two or more populations interbreed
Fossils & Evolution Ch. 2

30. Genetic variation: Alternation of generations in forams
“megalospheric”
(asexually produced)
“microspheric”
(sexually produced)
Fossils & Evolution Ch. 2

31. Ecophenotypic variation
Variation among individuals as a consequence of differences in their environments:
Nutrition
Exposure to sunlight (plants; animals with phtotsynthesizing symbionts)
Space (crowding)
Environmental stability
Fossils & Evolution Ch. 2

32. Sexual dimorphism in ammonoids
dimorphic
pair
dimorphic
pair
Fossils & Evolution Ch. 2

33. Fossil populations
Not as easy to work with as biologic (living) populations
Sources of difficulty
Sedimentary mixing (reworking; bioturbation)
Time-averaging; loss of temporal resolution
Preservation bias
Distortion
Dissolution (reduces observable variation)
Post-mortem sorting
Fossils & Evolution Ch. 2

34. Structural distortion of bivalve shapes
direction of
rock cleavage
undeformed
shape
Fossils & Evolution Ch. 2

35. Effects of selective post-mortem transport
Fossils & Evolution Ch. 2

36. Fossil populations (cont.)
Additional example of population “biasing” by selective transport
Devonian brachiopods
Leptocoelia (879 pedicle; 893 brachial)
Platyorthis (561 pedicle; 548 brachial)
Leptostrophia (378 pedicle; 35 brachial)
untransported, or not
selectively transported
selectively transported
Fossils & Evolution Ch. 2